Looking on the internet deeply has found these results:

I will call you on afternoon is the most popular phrase on the web.

I will call you in the afternoon 1162,000 results on the web	I will call you on afternoon 1254,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
I'll call you this afternoon once I put these together. Actually, I got a phone call this afternoon. We call this the afternoon delight room. I get a call yesterday afternoon from my cousin Eric. I waited for your call all afternoon. I had a telephone call this afternoon from the Governor of City Jail. If it will make you feel better, I will call him this afternoon and reiterate to him that he needs to stay away from you. I had an interesting call this afternoon from Dr. Frankel...	If it will make you feel better, I will call him this afternoon and reiterate to him that he needs to stay away from you. I was just out making my Sunday afternoon calls. I get a call yesterday afternoon from my cousin Eric. I had an interesting call this afternoon from Dr. Frankel... Actually, I got a phone call this afternoon. I'll call you this afternoon once I put these together. I waited for your call all afternoon. I had a telephone call this afternoon from the Governor of City Jail.

I will call you in the afternoon

1162,000 results on the web

I will call you on afternoon

1254,000,000 results on the web

More popular!

I'll call you this afternoon once I put these together.
Actually, I got a phone call this afternoon.
We call this the afternoon delight room.
I get a call yesterday afternoon from my cousin Eric.
I waited for your call all afternoon.
I had a telephone call this afternoon from the Governor of City Jail.
If it will make you feel better, I will call him this afternoon and reiterate to him that he needs to stay away from you.
I had an interesting call this afternoon from Dr. Frankel...

If it will make you feel better, I will call him this afternoon and reiterate to him that he needs to stay away from you.
I was just out making my Sunday afternoon calls.
I get a call yesterday afternoon from my cousin Eric.
I had an interesting call this afternoon from Dr. Frankel...
Actually, I got a phone call this afternoon.
I'll call you this afternoon once I put these together.
I waited for your call all afternoon.
I had a telephone call this afternoon from the Governor of City Jail.